1. Field of the Invention
The present invention relates to an image display apparatus and a method for controlling the image display apparatus.
2. Description of the Related Art
Image display apparatuses such as liquid crystal display apparatuses (LCD), plasma display apparatuses (PDP), field emission display apparatuses (FED), and organic EL display apparatuses (OLED) are available as flat panel display apparatuses (FPD).
In these FPDs, a large number of display elements must be formed on a substrate. A light emission characteristic of the display elements is affected by slight differences in manufacturing conditions and so on. Therefore, it is typically difficult to make the light emission characteristics of all of the display elements provided in the FPD perfectly uniform. Unevenness in the light emission characteristic causes brightness variation, leading to deterioration of an image quality. In the case of an FED, for example, surface conduction type electron-emitting devices, Spindt type electron-emitting devices, MIM type electron-emitting devices, and carbon nanotube type electron-emitting devices are used as electron-emitting devices. When differences occur in a shape or the like of the electron-emitting devices due to differences in the manufacturing conditions of the electron-emitting devices and so on, variation occurs in an electron emission characteristic of the electron-emitting devices. As a result, brightness variation occurs, leading to deterioration of the image quality.
In response to this problem, a constitution for correcting a video signal in accordance with the light emission characteristic of each display element has been proposed (brightness variation correction). For example, in one method, correction data including an adjustment ratio (a correction value) for reducing brightness variation are prepared in advance for each display element and the brightness variation is reduced by multiplying the adjustment ratio by an input video signal. However, the brightness variation may be dependent on a gradation value (the variation may be gradation-dependent). Therefore in order to decrease brightness variation for all the gradation values, a correction value corresponding to each gradation value must be provided.
Furthermore in the above mentioned brightness variation correction, a correction value corresponding to a video signal (gradation value) for the element is selected for each element constituting the pixel and the correction is performed. Therefore the correction values of all the gradation values must be read in advance from a memory (storage unit) storing the correction values.
This means that the processing band (processing band of a memory) which is required when correction data is read from the necessary memory increases as the performance of the display panel, such as gradation performance and definition, improves. In concrete terms, in the case of improving the display performance in a low gradation area (area in which gradation value is small), the brightness variation increases as gradation decreases, so more correction values must be provided for gradation values used for a low gradation area. This increases the capacity (processing capacity) of the correction data to be transferred (correction data which must be read in advance), and also increases the required processing band of memory. In order to support an ultra high definition standard, such as 4K2K for supporting digital cameras, from 2K1K, which is the current HDTV broadcasting standard, a higher definition is required, and the required processing speed (speed to read the correction values) increases in proportion to the increase of definition. Since the required processing band of memory is determined by processing capacity×processing speed, higher definition increases the required processing band of memory.
Possible methods for supporting this increase in processing band are increasing functions or increasing speed of memory (volatile memory). However if functions of a volatile memory are increased, cost increase or other problems occur due to the drastic increase in the number of pins of the LSI for controlling the volatile memory. In terms of increasing speed of a volatile memory, increasing speed exceeding a predetermined level is not easy, because device performance of the volatile memory is limited, and the degree of difficulty in substrate design increases.
If the capacity of the correction data is decreased in order to decrease the required processing band of memory, correction performance drops. In other words, the effect of decreasing processing band of memory and the effect of correcting brightness variation are in a trade-off relationship.
Hence improving display performance by an inexpensive system dramatically increases the value of FPD.
Available prior art on brightness variation correction are, for example, technologies disclosed in Japanese Patent Application Laid-Open Nos. 2000-122598, 2001-350442 and H11-202827, but the above mentioned problem cannot be solved by these technologies.
Both of the technologies of Japanese Patent Application Laid-Open Nos. 2000-122598 and 2001-350442 are technologies for decreasing gradation-dependent brightness variation, and are not for decreasing the required processing band of memory. In concrete terms, the technology disclosed in Japanese Patent Application Laid-Open No. 2000-122598 is a technology related to brightness variation correction using a correction value for each gradation value, and the technology disclosed in Japanese Patent Application Laid-Open No. 2001-350442 is a technology for providing correction values for specific gradations and calculating the correction value of a gradation value between the specific gradations using interpolation.
The technology disclosed in Japanese Patent Application Laid-Open No. H11-202827 is a technology for suppressing the generation of uneven color of longitudinal lines between blocks which are generated when correction values for a block is determined based on the pixel at the center of the block, and is not a technology for decreasing the gradation-dependent brightness variation.